cmd: add rng command

For the RNG uclass we currently only have a test working on the sandbox.

Provide a command to test the hardware random number generator on
non-sandbox systems.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index a6bfa87..7a78343 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1672,6 +1672,13 @@
 	  milliseconds. See also the 'bootstage' command which provides more
 	  flexibility for boot timing.
 
+config CMD_RNG
+	bool "rng command"
+	depends on DM_RNG
+	select HEXDUMP
+	help
+	  Print bytes from the hardware random number generator.
+
 # TODO: rename to CMD_SLEEP
 config CMD_MISC
 	bool "sleep"